Chop the sweet potatoes into small chunks. Peel the red onion and slice thinly. Peel the garlic clove and crush or finely grate.
Pour 1 tbsp oil into a pan and bring to a medium heat. Slide in the onion and garlic and sprinkle in a pinch of cayenne pepper (cayenne is HOT, so add a little to start with, then taste and add more if preferred) and a little salt and pepper. Cook for 3-4 mins, stirring occasionally, till softened.
Add the sweet potato pieces to the pan and fry for a further 2 mins.
Pour 800ml boiling water into the pan and swirl in the lentils. Crumble in the stock cube and stir well to dissolve. Return to the boil, then reduce to a simmer and cook gently for 15 mins, or till the sweet potato and lentils are tender.
Use a stick blender to whizz a little, still keeping a few chunks of vegetable if preferred. No stick blender? Simply transfer the soup to a food processor and blend (in batches if necessary).
Ladle the soup into bowls and serve with an extra pinch of cayenne and a good crack of pepper.